The invention relates to a battery for the production of electric energy by reaction between hydrogen peroxide (H2O2) or oxygen, and aluminium or lithium or a mixture thereof, and which utilizes electrodes of bottle brush shape, and which can mechanically be charged by inserting metal anodes. The advantages of the invention are a battery with high utilization of the reactants combined with the possibility of quick mechanical charging.
The main object of the invention is a battery which can be utilized for the energy supply of small unmanned underwater vehicles (UUV), but the invention is not limited for this use only. The battery is going to have pressure-compensated operation, i.e. the battery does not have to be encapsulated in a pressure tank. The cells are based on an alkaline electrolyte with H2O2 or oxygen as the oxidant, and metal anodes. The oxidant is added to the electrolyte, and the electrolyte is pumped through the cells in the battery. The anode material are alloys which form soluble reaction products by anodic dissolution in alkaline electrolyte.
xe2x80x9cBottle brushxe2x80x9d electrodes are known from NO 171 937, (Garshol and Hasvold), where there is described electrodes formed as bottle brushes. The purpose of these bottle brush electrodes are to obtain maximum area of the electrodes combined with good conductivity, low resistance to flow and a sufficiently mechanically stable solution.
Batteries which utilize oxygen or hydrogen peroxide, where a circulation of electrolyte takes place, are known from U.S. Pat. No. 4,305,999 (Zaromb). The anode is made of consumable metal, especially zinc, magnesium or aluminium. The purpose of the ""999 Patent is to regulate the electrolyte level in the battery cell in relation to the power consumption in such a way that unnecessary corrosion is prevented.
U.S. Pat. No. 4,910,102 describes a battery and a process for operating the battery, where bipolar electrodes are included consisting of an inert cathode which works as a hydrogen peroxide electrode, and an anode plate of aluminium, magnesium or alloys thereof. (In the abstract for U.S. Pat. No. 4,910,102 there seems to be an error: There is referred to a hydrogen electrode, but it seems to be a hydrogen peroxide electrode. Further there is referred to bipolar cathodes; the correct term seems to be bipolar electrodes). The electrolyte flows through the battery, and H2O2 is added in concentrations between 0.5% and 30% as volume part of the electrolyte. The electrolyte is, for example, sea water.
Expected time of discharge for such a battery for an unmanned underwater vehicle is long, typically more than 10 hours. The long time of discharge gives low current densities which subsequently allows for relatively large electrodes and a large distance between the electrodes in the cells of the battery. The battery comprises one or more cells. Anode materials of current interest are alloys which form soluble reaction products by anodic dissolution in an alkaline electrolyte. The rate of corrosion of the metal in the electrolyte has to be relatively low, which excludes the alkaline metals, except for lithium. Most appropriate are probably alloys of aluminium such as utilized earlier in FFI""s alkaline aluminium/air battery and as described in Hasvold: xe2x80x9cDevelopment of an alkaline aluminium/air battery systemxe2x80x9d. Chemistry and Industry (1988), pp 85-88, and Stxc3x8rkersen: xe2x80x9cDevelopment of a 120 W/24V Mechanically Rechargeable Aluminium-Air Battery for Military Applicationsxe2x80x9d. Power Sources 13, (1991), Ed.: Keily, T. and Baxter, B. W., pp 213-224.
Galvanic cells, which utilize hydrogen peroxide (HP) as oxidant (xe2x80x9ccathodic depolarizerxe2x80x9d), have been known for long. In some systems, HP is utilized directly in the cell, while in other systems, HP is used as a storage medium for oxygen, i.e. as an oxygen carrier. In the last case, one lets H2O2 decompose in a reactor and supplies the cells with oxygen from this reactor:
2H2O2=2H2O+O2xe2x80x83xe2x80x83(1)
The oxygen is consumed in a gas cathode in a fuel cell or in a metal/oxygen battery. A typical example of such a technology is Alupowers""s alkaline aluminium/oxygen battery for operation of unmanned underwater vehicles and is described in Deuchars, G. D. et al.: xe2x80x9cAluminium- hydrogen peroxide power system for an unmanned underwater vehiclexe2x80x9d Oceans 93 (1992), Vancouver, pp 158-165. In other cells, as e.g. described by Zaromb in U.S. Pat. No. 4,198,475, HP is added directly to the cathode in an aluminium/hydrogen peroxide battery with alkaline electrolyte. Whether HP decomposes in the electrolyte under formation of oxygen which in turn is reduced on the cathodes, or HP is reduced directly on the electrode surface, makes little difference in practice.
The advantage of utilizing HP as oxidant instead of oxygen is that the storage is substantially easier. Further, HP is miscible with water and can be added directly to the electrolyte in the desired concentration. In a UUV, the storage can also, if desired, be made outside the pressure hull. According to equation (1), 1 kg pure HP equals 0.471 kg oxygen. Pure HP implies a handling risk, as HP is unstable and the decomposition of HP releases a considerable amount of energy. This risk is considerably reduced by increasing the contents of water. 70% HP can be handled by attention to special precautionary measures, and at 50%, the heat of decomposition is no longer sufficient for complete vapourization of the water forming. In 70% HP, the xe2x80x9coxygen partxe2x80x9d of the weight constitutes approx. 33% and in 50% HP approx. 24%. Liquid oxygen, LOX, provides effective storage based on weight, but cryogenic storage of oxygen demands a certain thickness of the isolation, so that one for small systems gets very voluminous tanks in relation to the useful volume. The demand for insulation increases with the time the oxygen is to be stored. Further, a cryogenic storage tank is basically not suitable at large external pressures. For this reason, the storage in a UUV has in practice to be carried out in a pressure tank, which makes the system not very well suited for application in small vehicles.
The last and most common storage form for oxygen is under pressure in cylindrical- or spherical pressure tanks (bottles). This is very practical as long as the pressure of the battery is less than the bottle pressure, as the oxygen supply can be regulated by operating a valve. The tanks can be exposed to external pressure and be arranged on the outside of the pressure hull. Traditional metal tanks are heavy, typically an empty weight of 15 kg can store 4 kg oxygen, but fibre reinforced tanks can be made considerably lighter, and a storage capacity of 40-50% is not unlikely in the future.
At 300 bar, oxygen has a density of approx. 0.4 kg/litre and a system density by utilizing fibre-reinforced tanks of approx. 0.2 kg oxygen/litre. This provides somewhat more voluminous storage by utilizing pressure tanks than by oxygen storage in the form of 50% HP. Further, it has to be taken into consideration that with UUV batteries which operate at ambient pressure, the oxygen has to actively be pumped out of the bottles when external pressure is higher than the bottle pressure. This is a considerable problem for UUV""s which are to operate at great depths. In comparison, a HP storage will normally have ambient pressure independent of depth. Finally, it should be mentioned that while HP can directly be mixed in the electrolyte in the desired concentration, the solubility of oxygen in the electrolyte is low. Even if the solubility increases proportionally with pressure, the rate of dissolution is relatively slow, which can entail a complex system for the mixing of oxygen into the electrolyte. For the above mentioned reasons, one has primarily considered the use of HP in UUV batteries, but an oxygen-based battery which operates at a pressure of more than 5 to 10 bar, will have almost identical properties.
A problem with batteries where the oxidant is dissolved in the electrolyte is that the oxidant and anode metal can be consumed by direct reaction with each other. For Al this gives:
2Al+3H2O2+2OHxe2x88x92=2Al(OH)4xe2x88x92xe2x80x83xe2x80x83(2)
2Al+3/2O2+2OHxe2x88x92=2Al(OH)4xe2x88x92xe2x80x83xe2x80x83(3)
These non-current-producing reactions lead to losses of reactants and subsequently to reduced energy output from the cell. Further, they lead to an undesired heat generation in the cell. Reactions (2) and (3) also lead to a rise in anode potential, which reduces the cell voltage.
Both oxygen and H2O2 are strong oxidants which in strongly alkaline solutions very quickly react with anode materials of current interest, such as aluminium/tin alloys. By sufficiently high reactivity, the rate of reaction will be limited by the transport of oxidant to the anode surface (limiting current conditions). The rate of transport is given by the local hydrodynamic conditions. Hydrodynamic parameters which influence the limiting current are among others the character of the electrolyte flow (laminar/turbulent) and local velocity of flow and the physical dimensions of the anode. The rate of transport at limiting current will be close to being proportional to the concentration of oxidant. Thus, it is important to keep the concentration of oxidant in the electrolyte as low as possible. To reduce this parasitic reaction between oxidant and anode metal, it is common to use a membrane which separates the solution which surrounds the cathodes and which contains the oxidant (the catholyte) from the electrolyte surrounding the anodes (the anolyte). The loss according to (2) and (3) will then be reduced to the amount of oxidant which diffuses through the membrane.
FIG. 1 shows schematically a cell based on separate anolyte and catholyte, according to the related prior art.
The cell is composed of an anode chamber I with an anode II in an anolyte III. Between the anode chamber and the cathode chamber V, a membrane or separator IV which separates the anolyte III from the catholyte VII, but which allows transport of current (ions) through the membrane. The positive electrode in the cell, the cathode VI is made of an electrically conductive material which is a good catalyst, or is covered by a good catalyst, with a view to the reduction of hydrogen peroxide or oxygen or which catalyzes the decomposition of hydrogen peroxide followed by the reduction of oxygen.
The anode II can for example consist of pure aluminium alloyed with 0.1% tin, while the anolyte III can e.g. be 7 M KOH or NaOH. Also, the catholyte VII can consist of 7 M KOH, but will also contain oxidant (H2O2 or O2). For ensuring good stirring and thermal control, the electrolytes may be circulated between one or more cells and a reservoir. Further, the oxidant has to be supplied to the catholyte as it is consumed. The addition can be controlled by either a sensor on the oxidant in the electrolyte or from the calculated consumption from Faraday""s law and an estimate of the expected corrosion.
The cell according to the related art in FIG. 1 produces current by oxidation of aluminium at the anode II according to
2Al+8OHxe2x88x92=2Al(OH)4xe2x88x92+6exe2x88x92xe2x80x83xe2x80x83(4)
The electrons are consumed at the cathode VI according to
3H2O2+6exe2x88x92=6OHxe2x88x92xe2x80x83xe2x80x83(5)
either by direct reduction of HP, or by decomposition followed by the reduction of oxygen forming:                                           3            ⁢                          H              2                        ⁢                          O              2                                =                                                    3                2                            ⁢                              O                2                                      +                          3              ⁢                              H                2                            ⁢              O                                      ⁢                  
                ⁢                                                            3                2                            ⁢                              O                2                                      +                          3              ⁢                              H                2                            ⁢              O                        +                          6              ⁢                              e                -                                              =                      6            ⁢                          OH              -                                                          5b            
The sum of the anode and cathode reactions gives the cell reaction, i.e.
2Al+3H2O2+2OHxe2x88x92=2Al(OH)4xe2x88x92xe2x80x83xe2x80x83(6)
It appears from the equations that there is a consumption of both aluminium, hydrogen peroxide and hydroxyl ions when the cell delivers current. According to equation (6) and Faraday""s law, 9 grams of Al, 17 g HP and 62 g 7 M KOH are consumed to deliver 1 F current. This corresponds to 305 Ah/kg.
Unfortunately, in a real system, a substantially lower charge density is obtained. This is partly due to the fact that the entire alkaline electrolyte cannot be consumed, and partly due to parasitic reactions which lead to the consumption of reactants (corrosion reactions). Beside direct reactions between oxidant and aluminium according to equations (2) and (3), there is a reaction between aluminium and water under the generation of hydrogen:
2Al+3H2O+2OHxe2x88x92=2Al(OH)4xe2x88x92+3H2xe2x80x83xe2x80x83(7)
Also by this reaction there is a consumption of hydroxyl ions. In addition, this reaction leads, unlike the reactions (2) and (3), to the formation of hydrogen. Hydrogen which is not dissolved in the electrolyte, will form gas which has to be removed from the battery as it is formed. This is unlike oxygen, which can be removed by cathodic reduction in the battery as long as it is dissolved in the electrolyte. Especially at high pressures where the solubility of oxygen in the electrolyte is high, the oxygen will not contribute to the formation of gas in the battery. Notwithstanding this, an aluminium-based battery with an alkaline electrolyte, capable of functioning, has to have a system for the handling of gas being formed.
Generally, corrosion does not lead to reduced energy density until the corrosion current approaches the same order of magnitude as the average cell current. Further, it is evident that it is desired to keep the losses due to corrosion low, both for preventing loss of reactants, and also for reducing the problem with the formation of hydrogen in the battery.
The decomposition of water according to (7) can be kept at a low level by utilizing electrolytes and alloys with a very low impurity level. Further, the rate of reaction (7) is reduced by adding stannate to the electrolyte and by keeping a low temperature. These are techniques being well known from the work of the inventor and others on alkaline aluminium/air and aluminium/oxygen batteries. On the other hand, it is desired to operate at high temperature for reducing the polarization of the electrodes and to increase the conductivity of the electrolyte. Both these factors increase the cell voltage under load.
Depending among other things on the load, how well the cathode is catalyzed, the concentration of oxidant, the temperature and the aluminium alloy used, the typical cell voltage will be in the range of 1.2 to 1.6 V for such a cell under load. Other factors affecting the cell voltage are ohmic losses in the system, given by the current density, the geometry of the cell and the conductivity of the membrane. For keeping ohmic losses as low as possible, it is desired to minimize the distance between the anode and cathode. Further, one desires to have low resistance in the membrane separating the anolyte and catholyte. For a given membrane material, the membrane resistance will decrease with decreasing thickness, but, at the same time, losses of oxidant from the catholyte to the anolyte increas with increasing losses according to (2) and (3) as a result. Further, mechanical strength sets a lower limit for membrane thickness. An elegant solution where a porous cathode acts as separator between anolyte and catholyte is disclosed by C. L. Marsh et al in U.S. Pat. No. 5,445,905. Thereby the problem of voltage drop across the membrane which separates anolyte and catholyte is avoided. For preventing contact between anode and cathode, a coarse-meshed insulating net is utilized.
The advantages of a membrane-based system can be summarized by:
a) Low losses of capacity by direct reaction between oxidant and anode.
b) The concentration of oxidant can be kept high, which makes high current densities on the cathode possible.
c) Solid particles in the anolyte (such as Al(OH)3) does not affect the cathode reaction. while the disadvantages are
d) Complicated structure of the cells with separate circuits for anolyte and catholyte.
e) The membrane contributes to the ohmic losses in the battery.
f) Membranes are not very mechanically robust.
Especially for batteries for repeated use, the points d) and f) provide problems with reliability. Such batteries are mechanically charged by draining of the electrolyte, anodes are replaced, new electrolyte is filled up and container with oxidant is refilled. This leads to demands for easy disassembling and leak proof operation which can be difficult to satisfy.
The above mentioned disadvantages of the prior art are solved by utilizing a cell for the production of electric energy by reaction between hydrogen peroxide or oxygen, and aluminium or lithium or a mixture thereof, and hydroxyl ions in water, where the cathodes are cylindrical and based on radially oriented carbon fibres attached to a stem of metal. The novel feature of the invention is that the anodes and cathodes are arranged in a flowing electrolyte of KOH or NaOH dissolved in water, and where the electrolyte contains the oxidant in low concentration.
In the following, there will be given a detailed description with reference to the appended drawings which illustrate a preferred embodiment of the invention.